In recent years there has emerged a new class of small body worn devices that monitor one or more vital signs and transmit the readings wirelessly to a receiver unit without encumbering a patient with cables. For example, wireless and self-adhesive electrocardiography (ECG) monitors, which are placed on a subject to permit the remote monitoring of cardiac rhythm, are known e.g. from U.S. Pat. No. 3,943,918. A short-range wireless connection such as Bluetooth™ may be used. Typically the wireless receiver unit retransmits the data to an IT system for processing and display. The data (raw and/or processed) may be stored in a data base or electronic medical record. Within the IT system or individual patient record various “rules” may operate to alert medical staff when one or more of the vital signs moves outside the limits set for a patient.
In the context of cardiac monitoring using electrocardiography (ECG) electrodes, especially self-adhesive electrodes, wireless transmission of the raw ECG readings is possible but to transmit the entire ECG signal requires large amounts of bandwidth and consequently the monitoring device must be able to provide sufficient transmitting power. In order to alleviate such problems it has been proposed to set up the monitoring device so that the ECG signal is only sampled at pre-programmed intervals, for example once every 2, 5 or 10 minutes. The wireless power consumption scales proportionally with sampling rate. This enables the average heart rate to be calculated without requiring near-continuous transmission of data and thereby lowers the power consumption of the wireless transmitter. However this approach does not allow every heartbeat and its associated ECG peak to be recorded and potentially processed. Such a quasi-continuous measurement of heart rate may not reveal cardiac arrhythmias or other information useful for clinicians.
On the other hand, it has been proposed to perform full data processing on a wireless ECG sensor node rather than transmitting raw data. It has been reported, however, that in previous attempts the increased power consumption from local processing counteracts the limited savings in the radio power from a reduced rate of data streaming. There remains a need for an effective system for local ECG signal processing and data transmission.
It is also known to provide wireless heart rate monitors which are held against the patient by a chest strap. Using a chest strap for support means that it can be possible to mount a larger wireless transceiver including a data processor that is able to process and/or transmit instantaneous data relating to cardiac function. However, not only do such devices tend to be heavy and bulky but the degree of contact with the skin is generally poor and prone to motion artefacts. Moreover the need for a chest strap to be fitted around the subject means that such monitors may not be suitable in trauma situations where the subject is physically injured or disabled.
Monitoring devices that combine heart rate measurement with other vital signs, such as temperature and respiration rate, generally utilise separate sensors for each parameter and read each sensor sequentially. In addition to failing to provide continuous readings, such devices are also unable to provide concurrent data, for example simultaneous heart rate and respiration rate measurements.